The present invention relates to a method for quality control of components preferably manufactured in serial production and, in particular, a method for determining a selected machining quality (qualities) of components in accordance with the preamble of patent claim 1.
The quality of components manufactured by means of a manufacturing method strongly influences their use properties and service lives. This applies, in particular, to highly stressed components of, for example, a gas-turbine aircraft engine. In this case, the quality of the components relates, in particular, to their dimensional accuracy, their static and dynamic strength, and/or the surface quality in the machined regions.
The sought quality or machining quality of manufactured or machined components can be tested without destruction only with very great effort, with the testing operation generally occurring in time after the manufacturing process. For example, during metal cutting of workpieces by NC machines (drilling, milling, lathing, broaching, grinding), therefore, there exists the need, particularly in serial manufacture, to learn the result of the machining operation in quasi real time without interruption of the machining operation or the entire course of manufacture. However, without special measures and equipment systems, this information may be obtained only by time-consuming and cost-intensive measurements after the conclusion of the machining operation.
The use of such known equipment systems, such as, for example, photo cameras with automatic image processing, is problematic, because they do not have the requisite sensitivity (resolution) for important quality criteria, they represent an obstacle in the operating room of machines, and/or they require special maintenance by specially trained personnel. Of interest for monitoring a manufacturing method, therefore, are methods by means of which conclusions can be drawn in quasi real time about the quality of the manufactured components.
Fundamentally known from practice are methods for the quality-relevant monitoring of a manufacturing method, in which the signal curves of parameters of the manufacturing method are recorded by measuring techniques, wherein several signal curves that are recorded by measuring techniques that may be assigned to a correct and thus qualitatively high-grade manufacturing course are statistically analyzed in order to provide theoretical curves for the signal curves that can be recorded by measuring techniques. Then, for the quality-relevant monitoring of a manufacturing method, actual curves for the signal curves, which are recorded by measuring techniques, are compared with the theoretical curves determined beforehand for the respective manufacturing steps, whereby then, when the actual curves deviate from the theoretical curves by more than a defined measure, this points to the conclusion of a qualitatively poor manufacturing method and thus a quality defect in the manufactured component.
In more concrete terms, in accordance with the prior art, the manufacturing method that is to be monitored in terms of component quality or machining quality is divided into a sequence of individual steps or sections, with, for each individual step, at least one signal template being generated from the control commands of the individual steps and/or from the state variables recorded by measuring techniques.
These signal templates provide theoretical curves for signal curves or time series of parameters of the respective manufacturing method, these signal templates involving either theoretical curves that are determined by calculation, or theoretical curves generated beforehand from real signal curves by way of statistical analyses. This has the advantage that the method of calculation can then be employed for the quality-relevant monitoring of a manufacturing method when components having relatively small lot sizes are to be manufactured using the manufacturing method that is to be monitored, whereas the analytical provision of signal templates finds application more for serial manufacture. The overall course of operation described above finds application, in principle, also in the case of the present invention and is therefore fundamentally applicable to the method of determination according to the invention.
It has been found, however, that particularly the identification of manufacturing quality defects on the basis of signal templates gives rise to appreciable problems in practice. Thus, particularly in the case of complex manufacturing processes as well as when complicated machining tools are used, there is no causal relation between the machining quality to be determined and the signal templates or there is inadequate casual relationship, so that a subsequent correlation consideration during the analysis of the recorded signal curves does not provide clear and thus useable results.
Moreover, the machining qualities of interest at the sites of a component to be machined relate respectively to                their dimensional accuracy,        their surface quality,        and the quality of the respective edge-zone structure.        
According to a known method, in which, by means of sensors, certain operating and process parameters are measured in order to determine directly from them or by means of process models the corresponding characteristic values of the technological state of the machining process, largely numerical processing of sensor signals according to different mathematical methods and strategies are already used in diverse manner in real time or quasi real time. However, it is possible only to determine the instantaneous process state (see, for example, U.S. Pat. No. 5,070,655) or the state of wear of tools (see, for example, U.S. Pat. No. 5,251,144). Also known are corresponding methods and devices that, in this case, make do without additional sensors and use information that is acquired from already existing process variables, such as the power of the final drive (e.g., advance feed, spindle torque) without anything further (see for this, for example, U.S. Pat. No. 7,206,657). On the basis of the process states determined online in this manner, it is possible thereby to optimize the process by changing the process parameters. The optimization can take place with different goals, such as, for example, minimal tool wear or minimal machining time (see for this, in particular, U.S. Pat. No. 4,031,368). On account of the problems already discussed, however, an optimization directly according to the aforementioned three key machining qualities is not known.
In view of this prior art, it is a problem of the present invention to provide a method by means of which a determination of at least one of the machining qualities selected preferably from the three machining qualities mentioned is possible in quasi real time.